1. Field of the Invention
The present invention relates to a crack measuring method and apparatus for measuring the length and width of crack growth in a surface of a structure.
2. Description of the Related Art
In general, it is needed to measure the length of crack growth in the method for measuring physical properties of materials for various structure designs.
There are two types of conventional methods as follows. A first conventional method is the method how the worker observes and measures directly the length of crack growth in a surface of a structure. A second conventional method is the method for measuring automatically the length of crack growth and inspecting automatically crack growth by using an electrical signal. The conventional methods will be described with reference to FIG. 1 and FIGS. 2A and 2B.
FIG. 1 is a drawing illustrating a method for measuring the length and width of crack growth by using a ruler according to conventional techniques.
In the first conventional method how a worker observes and measures directly the length of crack growth in a surface of a structure, as shown in FIG. 1, in a case of a large crack, the worker contacts the ruler with the surface of a structure to measure the length of crack growth. Meanwhile, in a case of a small crack, the worker measures the length of crack growth in such a way that the worker magnifies and sees the small crack with a microscope, or in such a way that after storing the magnified image, the worker compares the stored magnified image with a reference scale grid. However, in the first conventional method, if crack rapidly grows or fine crack growth should be sensed in real time, a significant error occurs to measure the length of crack growth. Further, due to the worker's poor ability or subjective judgment, a deviation may be occurred.
FIG. 2A is a drawing illustrating a crack growth gauge attached to a cracked part in a surface of a structure according to conventional techniques. FIG. 2B is a drawing illustrating a resistance change sensed by the crack growth gauge according to conventional techniques.
In the second conventional method for measuring automatically the length of crack growth by using an electrical signal, as shown in FIG. 2A, thin electrically-conductive strips are attached to a cracked part at the same intervals. And then, as shown in FIG. 2B, the length of crack growth is measured with the crack growth gauge by sensing breaking of a copper line based on resistance change in the thin electrically-conductive strips. The crack growth gauge has been used and come into the market. ([Reference 1] U.S. Pat. No. 4,149,406 (RUSSENBERGER, M. E.) 1979. 04. 17, [Reference 2] PANKEVICIUS and SPICER. Technique and apparatus for automatic monitoring of crack propagation along glue lines. JOURNAL OF MATERIALS SCIENCE, 1990, Vol. 25, ISBN 0022-2461, pages 3079-3082, [Reference 3] Vishay Measurements Group, model TK-09-CPB02-005/DP).
Although the second conventional method solves various disadvantages of the first conventional method, because the thin electrically-conductive strips are attached to the cracked part, they block crack growth. Accordingly, it is difficult to measure accurate physical properties. This may not influence in a case of the large thickness of a structure or the large crack growth. However, as small-sized and lightweight structures have rapidly increased, cases where it is not possible to make a test specimen of a structure large occur frequently. Further, cases where it is not possible to attach a gauge may occur frequently due to the position of a crack and features of a structure.
Accordingly, since the second conventional method influences the crack growth of a structure, it results in inaccurate measurement results and can not be used in the structure to which a gauge can not be attached.